1. Field of the Invention
The present invention relates to a picture quality improving apparatus, and, more particularly, to a picture quality improving apparatus which can reduce deterioration of picture quality caused when subjecting a multiplexed signal to interpolation.
2. Description of the Related Art
For instance, noise reduction is one of picture quality improving techniques for use in video signal processing apparatuses such as video cassette recorders (VCRs). Many of recent video signal processing apparatuses employ digital signal processing for noise reduction. The digital noise reduction is executed by correlating video signals between fields, not between lines. First, a noise signal is detected by using the correlation of video signals between fields and non-correlation of a noise signal between fields. Then, this noise signal is subtracted from the original signal, thus reducing noise. To reduce the circuit scale, prior to noise reduction, a digital color difference signal sample may be multiplexed with a digital luminance signal sample obtained by YC separation and A/D conversion. In this case, since part of a luminance signal sample is thinned out by the multiplexing, an interpolation circuit is required to interpolate the lost portion of the luminance signal sample after noise reduction.
In general there are two interpolation techniques. According to the first technique, in order to interpolate lost portion of a luminance signal sample, those luminance signal samples preceding and succeeding the lost portion ar added together and then divided by two to obtain the average. The second technique is to use a delay circuit to delay a sample that is to be thinned out in a multiplex processing, then puts this sample in an interpolation circuit at a predetermined timing to thereby interpolate lost portion of a luminance signal sample.
Such interpolation circuits, however, have the following shortcomings. Due to the former interpolation circuit having a sort of a low-pass filter characteristic, a high-frequency components is reduced after interpolation. This may blur edge portions of a picture when a video signal represents a fine pattern. With the use of the latter interpolation circuit, when a residual image is produced in noise, reduction processing of a motion picture, the original sample free of a residual image is fit in lost portion, thus causing a vertical line pattern to appear on a picture.